The present invention relates to a process for fabricating a thin-film solar battery comprising a transparent dielectric substrate which carries a patterned thin semiconductor film sandwiched between patterned electrode layers.
Most thin-film solar batteries available today comprise series-connected unit cells. FIG. 1 shows a typical example of such solar batteries. More specifically, FIG. 1(a) is a plan view of a typical example of such a battery and FIG. 1(b) is a cross-sectional view looking in the direction of the arrow X.
FIG. 1 illustrates transparent dielectric substrate 1, usually in the form of a glass plate, carrying three transparent electrodes 21, 22, and 23, each being made of a film of indium tin oxide (ITO), SnO.sub.2, or a laminate of these two materials. the electrodes are formed by first depositing an appropriate transparent conductive film over the entire surface of the substrate 1 by electrode-beam evaporation or sputtering and then working the deposited layer in accordance with photolithographic processes to form patterns as delineated by the dashed lines in FIG. 1(a).
Then, three regions 31, 32, and 33 of a pin-junction type amorphous silicon (hereinafter abbreviated as a-Si) layer are formed on the transparent electrodes 21, 22, and 23, respectively, by the following procedures. First, the a-Si layer comprising a p-type layer about 100 A thick, an intrinsic layer about 0.5 .mu.m thick, and an n-type layer about 500 A thick are deposited over the entire surface of the substrate 1 so as to cover the transparent electrodes 21, 22, and 23. Then, the deposited a-Si layer is worked by photolithographic techniques to form patterns as delineated by the solid lines in FIG. 1(a). The patterns of the regions 31, 32, and 33 are similar to those of the transparent electrodes 21, 22, and 23 but are slightly displaced from the latter in the same direction (rightward in FIG. 1) so that the transparent electrodes 21, 22, and 23 are partly exposed.
In a similar manner, a thin metal film is deposited over the entire surface of the substrate by evaporation or any other suitable technique, and the deposited layer is patterned by photolithographic processes to form metal electrodes 44, 41, 42, and 43. In FIG. 1(a), the patterns of the electrodes are delineated by one-short-and-one-long dashed lines and hatched to emphasize their presence. If such patterned metal films, which are usually made of aluminum, are formed by etching with an acid such as phosphoric acid, nitric acid, or a mixed acid, nascent hydrogen will evolve and may etch not only the metal layer but also the underlying transparent electrodes. In order to avoid this problem, the acid etching may be replaced by plasma etching using a gaseous etchant such as Cl.sub.2, BCl.sub.3, or CCl.sub.4. As a consequence of the above procedures, the metal electrodes 44, 41, and 42 are contacted with the transparent electrodes 21, 22, and 23, respectively, and a thin-film solar battery is formed of unit cell 11, unit cell 12, and unit cell 13 which are connected in series. Unit cell 11 comprises the transparent electrode 21, a-Si layer 31, and metal electrode 41; unit cell 12 comprises the transparent electrode 22, a-Si layer 32, and metal electrode 42; and unit cell 13 comprises the transparent electrode 23, a-Si layer 33, and metal electrode 43.
The above fabrication process presents several disadvantages. First, patterning the transparent conductive film, a-Si layer, and metallic film by photolithographic procedures requires that the application and drying of photoresist films be effected in the batch process. In addition, as the area of the solar cell increases, the size and cost of the equipment necessary to permit a liquid photoresist to be uniformly applied and to perform exposure and post-bake with a photomask increase, with the attendant increases in the cost of the photomask.
In view of these problems, it has been proposed that patterning for providing transparent electrodes, a-Si regions, and metal electrodes be carried out by scribing with a laser beam. Laser scribing eliminates the application of a photoresist and the use of a photomask, and allows a large solar battery to be formed on a single substrate by continuous working. However, the laser beam is only capable of linear scribing. Therefore, substantial time and efforts are required for removing the transparent conductive film, the a-Si layer, and the metallic film from the entire surface of the region that surrounds the solar battery as indicated by numeral 10 in FIG. 1. The a-Si layer in the peripheral portion 10 is highly sensitive to the formation of defects which must be eliminated in order to avoid charge leakage between the transparent electrode and the metal electrode. In addition, markers must be formed in the peripheral region to ensure that the individual layers are correctly positioned during patterning by a laser beam. This problem requires that a special step be carried out to form opaque markers precisely at predetermined positions on the transparent substrate. As an alternative, a method has been used wherein the edges of the substrate are brought into alignment with those of the individual layers to be deposited. However, in the absence of any marker, the patterns often are not precisely aligned and shorting sometimes occurs between the transparent electrode and the metal electrode. This shorting obviously impairs the characteristics of the device.